mm-video: vdec : Avoid updating stride in flushing state

 -omx component is accessing the buffer after fill_buffer_done call
  on the client. This is to update the stride value for smooth
  streaming.
- During flushing scenario, buffers are returned to the client through
  fill_buffer_done call and client is expected to the free the buffers
  in the same context. Accessing the buffer in this scenario is leading
  to a crash during testH264_adaptiveSmallDrc.
- Fix is to avoid updating stride value when in flushing state.

  CRs-fixed: 651562

Change-Id: Ibe79fad264f42da8a356ef6b43793203035bced7